1. Field of the Invention
This invention relates to methods of and apparatus for controlling and synchronizing the output of a multiplicity of seismic sources utilized in seismic exploration. The invention provides for maintaining shot-by-shot quality control at a control station and continuously verifies similarity.
2. Description of the Prior Art
Seismic sources can be characterized either as single point sources or multiple point sources. The single point source is a single source of acoustic energy, either a continuous wave source or an impulsive source. In land operations the single point source is typically fired utilizing a radio control system. A master unit receives a fire command to initiate a start command to a recording system. There is then transmitted by radio a synchronizing code to a slave unit located at the source. Once this is accomplished, the master stops transmitting and begins receiving. The slave unit begins transmitting a tone and fires the source. Upon the receipt of a time break signal, usually from a time break sensor, the tone frequency changes. The master unit decodes the tone change at the time the source applied energy to the earth's surface and applies a field time break signal to the recorder.
Problems arise in the case where multiple sources are utilized where it is intended either that all the sources apply energy simultaneously to the earth's surface, or in sequential fashion to produce plane waves with some desired directivity.
Assumptions have been made in utilizing land sources such as land airgun sources, that once the sources have been synchronized and calibrated, for example, at the initial stage of conducting operations in the day, that thereafter all sources will simultaneously apply energy to the earth's surface and the energy output waveform or signature will be constant. These assumptions have proven to be erroneous.
It has been recognized in the utilization of airguns in marine operations, that the firing time of the airguns will vary during the course of an operation during the day. In marine systems when the recording equipment is on the boat, compensation is readily achieved inasmuch as each airgun in an array and its related sensor are direct wired to the recording boat. An operator can observe the actual time of opening of a valve in each airgun and to make compensating changes in firing time as by introducing delays such that each airgun fires at the desired instant of time with respect to the other airguns in the array.
The marine technique is not applicable to land operations since it is impractical to direct wire each of the land sources to the recording truck. The communication between the recording truck or master station to the sources or slave stations is conducted by a radio link. Since but one frequency is available for data communication between the recording truck and the sources in conducting commercial seismic exploration, one can readily see that it is impossible to receive time breaks simultaneously from all the sources. Accordingly, it has been the practice to receive a time break but from one source and assume that, having initially synchronized the firing time of each of the airguns, that the application of energy from the guns to the earth will take place a predetermined instant following the transmission of a firing command. This assumption has led to the production of undesirable data inasmuch as the application of energy to the earth is not simultaneous, but there exist time delays giving rise to phase shifts in the applied signals which distort the waveform, and in some instances give rise to cancellation of energy applied to the earth.
In accordance with the present invention, the problems introduced by the prior art techniques are avoided. The system enables the production of data useful in the field. Waveforms of the input energy at each source, as well as the time occurrence of that energy, or the onset of that energy, for each shot are made available for the processing of the data.